/usr/lib/swi-prolog/doc/Manual/option.html

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<h2 id="sec:option"><a id="sec:A.15"><span class="sec-nr">A.15</span> <span class="sec-title">library(option): 
Option list processing</span></a></h2>

<p><a id="sec:option"></a>

<dl class="tags">
<dt class="mtag">See also</dt>
<dd>
- <code>library(record)</code> <br>
- Option processing capabilities may be declared using the directive <a class="pred" href="predicate_options.html#predicate_options/3">predicate_options/3</a>.
</dd>
<dt class="tag">To be done</dt>
<dd>
We should consider putting many options in an assoc or record with 
appropriate preprocessing to achieve better performance.
</dd>
</dl>

<p>The <code>library(option)</code> provides some utilities for 
processing option lists. Option lists are commonly used as an 
alternative for many arguments. Examples of built-in predicates are <a class="pred" href="IO.html#open/4">open/4</a> 
and <a class="pred" href="termrw.html#write_term/3">write_term/3</a>. 
Naming the arguments results in more readable code, and the list nature 
makes it easy to extend the list of options accepted by a predicate. 
Option lists come in two styles, both of which are handled by this 
library.

<dl class="latex">
<dt><b>Name(Value)</b></dt>
<dd>
This is the preferred style.
</dd>
<dt><b>Name = Value</b></dt>
<dd>
This is often used, but deprecated.
</dd>
</dl>

<p>Processing options inside time-critical code (loops) can cause 
serious overhead. One possibility is to define a record using <code>library(record)</code> 
and initialise this using make_<var>&lt;</var>record<a class="pred" href="arith.html#>/2">&gt;/2</a>. 
In addition to providing good performance, this also provides 
type-checking and central declaration of defaults.

<pre class="code">
:- record atts(width:integer=100, shape:oneof([box,circle])=box).

process(Data, Options) :-
        make_atts(Options, Attributes),
        action(Data, Attributes).

action(Data, Attributes) :-
        atts_shape(Attributes, Shape),
        ...
</pre>

<p>Options typically have exactly one argument. The library does support 
options with 0 or more than one argument with the following 
restrictions:

<p>
<ul class="latex">
<li>The predicate <a class="pred" href="option.html#option/3">option/3</a> 
and <a class="pred" href="option.html#select_option/4">select_option/4</a>, 
involving default are meaningless. They perform an <code>arg(1, Option, Default)</code>, 
causing failure without arguments and filling only the first 
option-argument otherwise.
<li><a class="pred" href="option.html#meta_options/3">meta_options/3</a> 
can only qualify options with exactly one argument.
</ul>

<dl class="latex">
<dt class="pubdef"><span class="pred-tag">[semidet]</span><a id="option/3"><strong>option</strong>(<var>?Option, 
+OptionList, +Default</var>)</a></dt>
<dd class="defbody">
Get an <var>Option</var> Qfrom <var>OptionList</var>. <var>OptionList</var> 
can use the Name=Value as well as the Name(Value) convention.
<table class="arglist">
<tr><td><var>Option</var> </td><td>Term of the form Name(?Value). </td></tr>
</table>
</dd>
<dt class="pubdef"><span class="pred-tag">[semidet]</span><a id="option/2"><strong>option</strong>(<var>?Option, 
+OptionList</var>)</a></dt>
<dd class="defbody">
Get an <var>Option</var> from <var>OptionList</var>. <var>OptionList</var> 
can use the Name=Value as well as the Name(Value) convention. Fails 
silently if the option does not appear in <var>OptionList</var>.
<table class="arglist">
<tr><td><var>Option</var> </td><td>Term of the form Name(?Value). </td></tr>
</table>
</dd>
<dt class="pubdef"><span class="pred-tag">[semidet]</span><a id="select_option/3"><strong>select_option</strong>(<var>?Option, 
+Options, -RestOptions</var>)</a></dt>
<dd class="defbody">
Get and remove <var>Option</var> from an option list. As <a class="pred" href="option.html#option/2">option/2</a>, 
removing the matching option from <var>Options</var> and unifying the 
remaining options with <var>RestOptions</var>.</dd>
<dt class="pubdef"><span class="pred-tag">[det]</span><a id="select_option/4"><strong>select_option</strong>(<var>?Option, 
+Options, -RestOptions, +Default</var>)</a></dt>
<dd class="defbody">
Get and remove <var>Option</var> with default value. As <a class="pred" href="option.html#select_option/3">select_option/3</a>, 
but if <var>Option</var> is not in <var>Options</var>, its value is 
unified with
<var>Default</var> and <var>RestOptions</var> with <var>Options</var>.</dd>
<dt class="pubdef"><span class="pred-tag">[det]</span><a id="merge_options/3"><strong>merge_options</strong>(<var>+New, 
+Old, -Merged</var>)</a></dt>
<dd class="defbody">
Merge two option lists. <var>Merged</var> is a sorted list of options 
using the canonical format Name(Value) holding all options from <var>New</var> 
and <var>Old</var>, after removing conflicting options from <var>Old</var>.

<p>Multi-values options (e.g., <code>proxy(Host, Port)</code>) are 
allowed, where both option-name and arity define the identity of the 
option.</dd>
<dt class="pubdef"><span class="pred-tag">[det]</span><a id="meta_options/3"><strong>meta_options</strong>(<var>+IsMeta, 
:Options0, -Options</var>)</a></dt>
<dd class="defbody">
Perform meta-expansion on options that are module-sensitive. Whether an 
option name is module-sensitive is determined by calling <code>call(IsMeta, Name)</code>. 
Here is an example:

<pre class="code">
        meta_options(is_meta, OptionsIn, Options),
        ...

is_meta(callback).
</pre>

<p>Meta-options must have exactly one argument. This argument will be 
qualified.

<dl class="tags">
<dt class="tag">To be done</dt>
<dd>
Should be integrated with declarations from
<a class="pred" href="predicate_options.html#predicate_options/3">predicate_options/3</a>.
</dd>
</dl>

</dd>
</dl>

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swi-prolog-nox 6.6.4-2ubuntu1 / usr / lib / swi-prolog / doc / Manual / option.html
